In order to control the motion of an elevator car in the hoistway, precise and reliable measurements of its position and speed are essential. Conventionally, an incremental encoder or a series of switches in the hoistway are used to determine position and speed of an elevator car.
One new option is the use of a laser for measuring distance based on triangulation, measurement of diffraction, measurement of interference, or measurement of transit time. Applied at distances ranging from 1 to 100 meters or more, these methods have disadvantages which make their use for an elevator difficult and expensive. The requirement of a long coherent laser beam, the difficulty in measuring extremely short transit times related to the travel of the light, and the ambiguity of resulting patterns are inherent in those methods. In addition, the installation of a transmitter, receiver or reflector on the car creates the serious technical difficulty of aiming and reflecting a thin laser beam from a laterally moving and swaying surface over distances up to 100 meters.